1. Field of the Invention
The present invention relates generally to a control apparatus for a vehicular 4-wheel drive system, and more particularly to a control of a 4-wheel drive system of a vehicle provided with primary drive wheels and auxiliary drive wheels.
2. Description of Related Art
There is known a vehicular 4-wheel drive system provided with: primary drive wheels to which a drive force is transmitted from a drive power source; auxiliary drive wheels to which the drive force is selectively transmitted; a transfer device configured to distribute a torque from the drive power source to the primary drive wheels and the auxiliary drive wheels; and a coupling device configured to adjust the drive force to be transmitted to the auxiliary drive wheels. JP-2009-208633 A1 discloses an example of such a vehicular 4-wheel drive system, wherein an amount of torque to be transmitted through the coupling device is increased to a predetermined value in an engaged state of a lock-up clutch incorporated in a torque converter disposed between the drive power source and a power distributing mechanism, so that backlashes of gears provided in a power transmitting path from the coupling device to the auxiliary drive wheels are reduced to reduce a rattling noise generated due to abutting contacts of the gears upon variations of the torque transmitted through the power transmitting path.
By the way, there is proposed a vehicular 4-wheel drive system provided with a first clutch configured to selectively place a first power transmitting path between the above-indicated drive power source and the above-indicated auxiliary drive wheels, in a power transmitting state and a power cutoff state, and a second clutch disposed in a second power transmitting path between the first clutch and the auxiliary drive wheels, to selectively place the second power transmitting path in a power transmitting state and a power cutoff state. In a 2-wheel drive mode of this vehicular 4-wheel drive system in which the drive force is transmitted from the drive power source to the primary drive wheels, the first clutch and the second clutch are placed in released states, to prevent dragging rotary motions of rotary members disposed between the first clutch and the second clutch. Described more specifically, the second clutch takes the form of a coupling device disposed on an input side of a rear-wheel differential device, as disclosed in the above-identified publication JP-2009-208633 A1, a coupling device disposed in one of right and left rear-wheel drive axles, or a pair of coupling devices disposed in the respective right and left rear-wheel drive axles. The vehicular 4-wheel drive system may be provided with another coupling device in addition to the second clutch. In any one of the arrangements of the vehicular 4-wheel drive system described above, the torque to be transmitted through the coupling device is adjusted in an engaged state of the first clutch, to reduce the noise generated due to variation of a torque of the drive power source or a transmission, in the 2-wheel drive mode in which the first and second clutches are placed in the released states. Accordingly, the rotary members disposed between the first and second clutches are rotated at the same speeds as in the engaged state of the first clutch, resulting in deterioration of fuel economy of the vehicular 4-wheel drive system due to the rotary motions of those rotary members.